In order to implement simultaneous bidirectional communications (duplex transmissions) in a mobile communication system, a downlink channel from a base station to a mobile station (for example, a mobile phone) and an uplink channel from a mobile station to a base station are necessary. As for duplex schemes, there are two, that is, the FDD (Frequency Division Duplex) scheme in which uplink and downlink are separated by frequency and the TDD (Time Division Duplex) scheme in which uplink and downlink with the same frequency are separated by time.
In addition, the uplink channel and the downlink channel consist of frames, where one frame is divided in time, and constructed by plural timeslots (abbreviated as a slot hereafter).
Now, an overview of the aforementioned FDD scheme and the TDD scheme is provided referring to FIGS. 1A and 1B, and an overview of the FDD/TDD scheme adopted in PDC (Personal Digital Cellular), which has been commercialized in Japan, and in GSM (Global System for Mobile communications), in use in Europe is provided.
First, an overview of an FDD scheme is provided referring to FIG. 1A. FIG. 1A shows an example of frame configuration in the FDD scheme.
The FDD scheme is adopted in IMT-2000 (the third generation mobile communication system) represented in W-CDMA or cdma2000, in which the transmission (the reception) of the uplink slot during the transmission (the reception) of the downlink slot is possible because frequencies in up and down directions (f1, f2) are different from each other.
In a mobile communication system in which the FDD scheme is used, a communication band for the downlink channel from the base station to mobile station and a communication band for the uplink channel from the mobile station to the base station are separated from each other, so the separation between the up and down signals is implemented by providing the transmission and reception sections of the base station and the mobile station with filters (duplexers) for preventing interference between the transmit signal and the receive signal.
However, because the duplexer is an analog circuit, it is difficult to be miniaturized, leading to an obstacle to miniaturization of mobile telephones.
Next, an overview of the TDD scheme is provided referring to FIG. 1B. FIG. 1B shows an example of frame configuration in the TDD scheme.
The TDD scheme, which has been adopted in mobile communication systems such as PHS, doesn't require the duplexer because transmissions in the uplink and the downlink are performed at one frequency (f0). However, in the TDD scheme the transmission cannot be performed during the reception. Further, since the same frequency is used in the uplink and the downlink, it is necessary to maintain synchronization between the respective base stations. Thus, such a cellular system supporting an area ranging from several hundreds of m to several km tends to have a drawback in that it is easily subjected to influence of propagation delay, etc.
Next, an overview of the FDD/TDD scheme is provided referring to FIG. 1C. FIG. 1C shows an example of frame configuration in the FDD/TDD scheme.
The FDD/TDD scheme, in which separate frequencies are used for the transmission and reception and the transmission timing is different from the reception timing, doesn't require a duplexer. This is based on the fact that the PDC and GSM are systems predicated on line switching such as voice communication, and that the transmission timing and the reception timing are periodically constant.
By the way, it is believed that the leading requirement for future service will change from voice calls to data communication (packet transmission) such as on the Internet. In such a case, characteristics of the traffic can be expected to be such that the uplink channel will be used only for requesting information while the downlink channel is used for transferring massive amounts of data compared with the uplink channel, such as music, image, and picture. In other words, there is a need for a duplex scheme suited for Internet access with asymmetric traffic.
For this reason, a communication system using a CDMA-TDD scheme is proposed, which can accommodate the communication of the asymmetric information quantity of up and down lines effectively by controlling the number of time slots to be allocated for the respective up and down lines according to the total sum of the information quantity of the respective up and down lines (See, for example, JP11-234242 A publication.).
Further, from a similar viewpoint, a mobile communication system using the TDD scheme is proposed, which can perform smooth time division duplex communication by changing the slot interval ratio of an up line and a down line between a base station and a mobile station (See, for example, JP8-186533 A publication.).
As mentioned above, the conventional FDD/TDD scheme is suitable for line switching, that is, the communication in which information quantity is the same bidirectionally, such as voice communication, and in the conventional FDD/TDD scheme timing of transmission and reception is fixed in the system. Therefore, for asymmetric communications in up and down directions, there is a problem in that a flexible slot allocation cannot be implemented in the conventional FDD/TDD scheme.
In addition, according to the aforementioned publications, in the TDD scheme an effective accommodation is enabled by allocating time slots to the respective up and down lines according to the information quantity of the respective up and down lines, or in the TDD scheme frequency utilization efficiency is improved by changing the slot interval ratio of an up line and a down line according to the difference in information quantity between up and down lines; however, no description concerning the slot allocation in the case of the FDD/TDD scheme can be found in these publications.
On the other hand, since miniaturization of size in device-scale as well as improved performance are demanded for the fourth generation mobile communication system, a FDD/TDD scheme which doesn't require duplexers is expected to be developed as a duplex scheme which enables miniaturization of the equipment.